R. S. Day, III, D. Yarosh, and others have shown that approximately 20% of human tumor lines and viral transformed lines are hypersensitive to alkylating agents due to an apparent absence of Alkylguanine Alkytransferase. We have been able to partially purify the enzyme from human liver. This enzyme removes alkylation damage at the 06 position of guanine but not at other sites in DNA. We plan to further characterize this enzyme and purify it to the point where amino acid sequence information in obtained. In collaboration with D. Yarosh, human liver extracts are being used to isolate adequate quantities of this protein. Differences in the transcription of specific RNAs in mer+ and mer- cell lines were investigated. With hybridization subtraction, a cDNA library was constructed from a mer+ cell which has been depleted of sequences common to matched mer-cell lines. However, when other mer+ and mer- lines were used, this correlation was not consistent - i.e., none of the cDNA clones coded from a transcript consistently more abundant in mer+ compared to mer- cell lines.